This invention relates to a process for the preparation of olefin polymers in which a particulate multisite catalyst or catalyst mixture is used, in particular to a continuous polymerisation process employed to prepare particulate polymers and to the olefin polymers produced thereby.
The preparation of polymers can be achieved by the use of a variety of particulate multisite catalysts or catalyst mixtures as is well known in the art. The active catalysts may be, for example, mixtures between Ziegler-Natta, xcex7-liganded and chromium oxide type catalysts. Typically, such polymer preparations may be performed by gas phase or slurry polymerisation. In such polymer preparation, each type of catalytic site makes a polymer component with varying characteristics depending on the reactor conditions as well as on the type of active site.
Such polymerisations are relatively sensitive and may therefore be easily disturbed or disrupted so that a consistent polymer product is not produced. Disturbances may detrimentally influence the ratio between the polymer components produced by the catalysts causing inconsistencies in the final polymer product. This problem is most noticeable in the preparation of bimodal and multimodal polymer products.
There thus remains a need for an olefin polymerisation process which produces a consistent polymer product and a process by which inconsistencies in the polymer product can be detected during the actual polymerisation and the ratio of polymer components adjusted accordingly to eliminate such inconsistencies.
Many of the manipulable polymerisation parameters such as comonomer concentration, monomer concentration, hydrogen concentration and reactor temperature are manipulated to alter other properties of the polymer product such as melt flow rate, crystallinity or molecular weight distribution or in order to achieve optimum production economics by high production rate and low catalyst consumption. Such parameters are therefore unsuitable for manipulation to ensure a consistent polymer product.
It is proposed in WO96/07478 to control the molecular weight of a bimodal resin by catalyst manipulation using a catalyst comprising a bimetallic catalyst and a make-up catalyst. The make-up catalyst consists of a single metallic component. A process is described in which is employed a bimetallic catalyst and a make-up catalyst in which these catalysts are fed into the reaction vessel by a system of two separate catalyst feeders.
However, the method described in WO96/07478 may produce a polymer powder which is very inhomogeneous. Such inhomogeneities may be detrimental to the further use of the polymer, causing problems in processes if forming finished articles directly from powder or when the powder is extruded before further use in which case the resulting granules themselves become partially inhomogeneous.
Moreover, the major portion of the polymer made by the process described in WO96/07478 is prepared from the dualsite catalyst. Only minor amounts of the make-up catalyst are employed. This causes significant process difficulties since in a standard polymer manufacturing plant the feeders are all of the same dimensions. Thus, in this case whilst one feeder will operate at approximately the normal catalyst feed rate, the other feeder will operate at a much lower feed rate than it was designed for. Having such a small amount of catalyst in a feeder leads to severe difficulties in controlling the feed rate since leakage past the control system becomes a significant hazard. It is therefore advantageous if all feeders contain a significant amount of catalyst.
We have now found that we can overcome the problem of inhomogeneity without any feeders operating at lower catalyst feed rates by feeding particulate multisite catalysts or catalyst mixtures with rather small compositional differences between them using several feeders.
Moreover, we have now found a method for maintaining consistent polymer component ratios and for producing a homogeneous polymer product comprising feeding a catalyst composition having at least two catalytic sites from at least two different catalyst feeders substantially simultaneously into the reaction vessel. If any deviation in the ratio of the produced polymer components is detected then the ratio of the feed rates of the catalyst composition may be adjusted accordingly to restore the ratios to their desired values.